Utilization of digital communication techniques to communicate information between a sending station and a receiving station has become increasingly popular in recent years. Radio communication systems, such as cellular communication systems, are exemplary of communication systems which are increasingly constructed to utilize digital communication techniques.
Communication channels formed between the sending and receiving stations of a radio communication system are defined upon a portion of the electromagnetic spectrum, the "bandwidth", allocated to the system. The channels are defined, at least in part, upon carrier frequencies, "carriers", within the allocated bandwidth. The bandwidth allocated, and available, to a radio communication system is usually limited. And, the communication capacity of the radio communication system is defined, inter alia, by, and limited by, the bandwidth allocated to the system.
In a multi-user radio communication system, such as a cellular communication system, communication capacity limitations sometimes prohibit additional users from utilizing the communication system as a result of bandwidth limitations. By utilizing more efficiently the bandwidth allocated to the communication system, the communication capacity of the system can be increased.
The bandwidth allocated to a radio communication system can be more efficiently utilized if digital communication techniques are used to transmit information-containing communication signals upon channels forming links between a sending station and a receiving station.
When a communication signal is formed utilizing a digital communication technique, an information signal is sometimes digitized and modulated upon a carrier utilizing a selected modulation technique, such as, for example, a QPSK (Quadrature Phase Shift Keying) or a GMSK (Gaussian Minimum Shift Keying) technique. Use of other modulation techniques are sometimes alternately utilized. Because the information signal is digitized, the communication signal formed therefrom can be transmitted by a sending station upon a communication channel in discrete bursts. When the communication signal is transmitted in discrete bursts, the bursts are concatenated theretogether at the receiving station.
Because communication signals can be transmitted in discrete bursts, time division multiplexing of a carrier is permitted. Two or more channels can be defined upon a single carrier.
In at least one type of cellular communication system, a system constructed pursuant to the operational specification of the Global System for Mobile communications (GSM), a digital communication technique is utilized. Carriers of the bandwidth allocated to the communication system are divided into eight time slots. Eight-way time division multiplexing is provided in such a communication system, and bursts of communication signal portions are transmitted between a sending station and a receiving station on selected ones of such time slots. Carrier/time slot combinations form the communication channels upon which the communication signals are transmitted.
Standard protocols set forth in the GSM operational specification, define the structure of normal bursts communicated during time slots defined in the GSM system. The communication signal portions transmitted during the time slots defined in the GSM system are divided at least into a data field and a training sequence field. The training sequence field is formed of a series of bits, known to the receiving station. The purpose of transmitting known bits to the receiver is to allow the receiver to equalize the channel. Typically, the signal is distorted when it propagates through the radio medium and the equalization allows for the receiver to estimate the channel impulse response, i.e., how this distortion has affected the signal during its transmission to the receiver.
Such training sequence bits are utilized at the receiving station to facilitate detection of the informational content of the data fields communicated together with the training sequence field.
Cellular communication systems, both those utilizing conventional analog techniques and also those utilizing digital communication techniques, define cells throughout a geographical area encompassed by the cellular communication system. Collections of cells form cell clusters. In each cell cluster, the total available bandwidth allocated to the communication system is utilized. In successive cell clusters, the allocated bandwidth is reused. The communication capacity, limited by the number of channels which can be defined upon the allocated bandwidth, is effectively increased by reusing the channels in each of the cell clusters.
A problem sometimes associated with reuse of the bandwidth is co-channel interference. When communication signals are transmitted concurrently in different cells upon the same communication channel, such concurrently-transmitted signals sometimes interfere with one another; such interference is referred to as co-channel interference. Co-channel interference makes detection of the wanted-signal received at a receiving station more difficult. If levels of co-channel interference are significant, the quality of the signal detection might be inadequate.
Receiving stations which receive communication signals generated utilizing digital communication techniques sometimes include equalizer circuitry to facilitate signal detection of the informational content of a communication signal received at the receiving station. Typically, the training sequence forming a portion of a communication signal is utilized by the equalizer to facilitate the detection of the informational content of the wanted-signal received at the receiving station.
When the receive signal received at the receiving station is formed of both a wanted-signal component and also an interfering-signal component, an equalizer can be constructed to jointly detect both the wanted-signal component and the interfering-signal component. In such an equalizer, however, the training sequences associated with both the wanted-signal component and the interfering-signal component must typically both be known. While the training sequence associated with the wanted-signal component is typically known to the receiving station, the training sequence associated with an interfering-signal component portion forming at least a portion of the interfering-signal component is not necessarily and, typically is not, known to the receiving station. Without knowledge of the training sequence of the interfering-signal component portion, existing receiving stations are typically unable to properly detect and suppress such an interfering-signal component portion of a receive signal.
A manner by which to permit a receiving station to determine better the interfering-signal component portion of a receive signal received at the receiving station would be advantageous. By better detecting the interfering-signal component portion, suppression of such interfering-signal component portion can be better effectuated. Thereby, bandwidth reuse can be made more efficient, resulting in increased communication capacities of the communication system.
It is in light of this background information related to digital receivers that the significant improvements of the present invention have evolved.